Total Synthesis via Biomimetic Late-stage Heterocyclization, Assignment of the Relative Configuration and Biological Evalu-ation of the Nitraria Alkaloid (±)-Nitrabirine

Abstract

The racemic total synthesis of nitrabirine (5) together with its previously undescribed epimer 2-epi nitrabirine (5’) is accomplished via a six-step route based on a biomimetic late-stage heterocyclization. This allowed the assignment of the relative configuration of nitrabirine by the lanthanide-induced shifts (LIS) experiment, later on confirmed by X-ray diffrac-tion of obtained single crystals. Furthermore, oxidation studies demonstrated that the direct N-oxidation of nitrabirine does not yield nitrabirine N-oxide as reported earlier. In contrast, the reaction of hydrogen peroxide with nitrabirine (5) yields the salt 24’ whereas, under the same conditions, 2-epi nitrabirine (5’) surprisingly leads to a beforetime uncharac-terized product 22. Finally, a Fischer-Indole reaction gave access to novel tetracyclic nitrabirine derivatives 26a-d. A comprehensive biological evaluation of nitrabirine (5), 2-epi nitrabirine (5’), and all derivatives synthesized in this study revealed general biofilm dispersal effects against Candida albicans. Moreover, specific compounds showed moderate antibacterial as well as potent cytotoxic activities.